1. Field of the Invention
This invention pertains generally to circuit modules and components used as sub-assemblies in the assembly of complex electronic circuits. More specifically, the invention pertains to methods of reliably forming low-cost, high performance, compact electrical interconnections between the sub-assembly and larger electronic circuits.
2. Description of the Related Art
Several goals and objectives are pervasive throughout the electronics industry. Among these are a need for smaller sizes, higher reliability, lower power consumption, lower cost, and faster operating speeds. Many applications are awaiting or would most preferably use a circuit which is small and compact enough to be unobtrusive, while simultaneously offering improved operational capability. However, various ones of these goals are often in opposition to others. For example, size and capability are generally in conflict, and a circuit designer must choose whether a size needed for a given capability is appropriate. If not, the designer must choose what functions or capabilities can be sacrificed in order to keep the desired size. Similar design comparisons and trade-offs must be made with cost, yield, reliability, and so on. As a result, there is a continuing need to develop ways of constructing smaller, more efficient circuits that are still produced with high yields and moderate cost.
In pursuit of these underlying goals, modern circuits are constructed from a number of various sub-assemblies. Each of the sub-assemblies may be tested prior to assembly into the larger circuit, or, alternatively, may be assembled through a removable connection, so that when a sub-assembly is defective or fails, it may be replaced. Even the sub-assemblies are frequently assemblies of smaller devices or components. A primary reason for using sub-assemblies is the underlying device reliability, and the number of devices found in many of today's circuits. Manufacturing yields are dependent upon the reliability of the components used in the assembly of the product. In production of a circuit with thousands of parts, even if the individual components all have a failure rate of only one in one thousand, which is ordinarily considered reliable, the device built from thousands of these components will almost certainly fail, because at least one of the thousands of parts will be a bad one. Consequently, the manufacturer must choose between higher reliability components, which are sold at higher prices, and lower production yields.
In addition to yields, other challenges are significant. The electrical interconnects between a component or sub-assembly and other components or sub-assemblies will have a very significant impact on both the performance and size of the circuit and also upon the ease or difficulty of manufacture and repair. Many different methods have been proposed for making these connections. Yet, as before, each suffer from limitations in one area or another. As circuitry has been developed which operates at higher speeds, there has been a need for making electrical interconnections which are able to carry these high speed signals without significant degradation. Unfortunately, as signal frequencies increase, the length of an electrical conductor becomes very consequential to important characteristics of the conductor, such as cross-talk and line impedance.
To shorten the length of an interconnect, a configuration referred to as fine-pitch has been developed. A fine pitch interconnection will, for the purposes of this disclosure, be defined as an interconnection having spacings of 0.050 inches or less. In the prior art, standard interconnect spacings were at 0.100 inches, which at the time met the requirements of the available circuitry. However, with newer, faster circuits operating at higher frequencies, and with the desire to reduce both cost and size of circuit boards and other device surface area, there is frequently a need for tighter spacings, as is found now in fine pitch spacing.
The fine pitch interconnection, however, necessitates altering previous attachment techniques such as soldering. Previously, wave soldering, IR reflow, hot bar, and other similar techniques were used to make interconnections. Unfortunately, as the pitch decreases, meaning more electrical interconnects in a given space, the more likely solder is to bridge gaps between interconnects. If solder does bridge a gap, an electrical short circuit will result that can easily disable or destroy the functionality of the assembly.
As an alternative to the previous soldering techniques, a method was developed that uses solder balls to form reliable, controlled height electrical interconnections between a component and another component or a circuit substrate. In this method, a solder ball preform is placed onto an electrical connection point or pad. During a subsequent heating phase, the solder melts and reflows, attaching the solder ball to the electrical connection point. The component that now has a solder ball attached to and extending from the electrical connection point may then be placed so that the solder ball extends onto a second electrical connection pad of another device or substrate. The solder is again heated to reflow, leaving a small gap roughly equal to the diameter of the solder ball between the two components. The small gap may be controlled to some extent by use of the solder ball, and the amount of solder applied thereto is very precisely controlled.
The gap may serve a number of different purposes, among the most mundane which is to allow the subsequent cleaning of any flux or other corrosive residue that might have been released during the solder process. During operation of the devices, the solder also acts a small metal bridge. As is apparent, the small metal bridge does offer some resilience, and so any differences in expansion or contraction of the components during operation will, most preferably, be absorbed within the solder ball interconnect. Once two devices have been attached in this manner, they may be treated as a single unit. Nevertheless, if there should arise a need to separate the two components, the solder junction between the components may be melted non-destructively, thereby allowing assembly and, as needed, disassembly.
Unfortunately, in spite of the many advantages of these solder ball interconnects, the balls are quite small and difficult to handle. Production of one or a few parts in a lab is readily achieved, but volume production is much more difficult. Once again, yields of production directly affect manufacturing costs, and the equipment to handle these solder balls has not met the needs of the industry.
Several proposals have been made for handling these solder balls in a production environment, two examples which are illustrated by Bailey et al in U.S. Pat. No. 4,558,812 and Wilson et al in U.S. Pat. No. 5,284,287, the contents which are each incorporated herein by reference. These systems rely upon vacuum to locate the solder balls prior to attachment to a component or subassembly. Since the solder balls are very small, 20-40 thousandths of an inch as referred to by Wilson et al, and potentially smaller in the present invention, the holes that permit passage of vacuum must be smaller yet. Unfortunately, these holes are extremely difficult to clean. Yet, when even a single hole plugs, an interconnect will fail. One of the concerns in fact noted by Wilson et al is the issue of contamination of the vacuum pick-up tool with flux. The size limitation of the vacuum line, and yet the necessity of having it, has prevented the implementation of these solder balls in numerous applications where otherwise the balls would be advantageous.